A self-running construction vehicle, such as a wheel loader, includes a steering actuator and can change the running direction of a vehicle main body thereof by supplying operating oil to the steering actuator. The steering actuator is connected to a hydraulic pump through a steering control valve. The steering control valve changes the direction of the operating oil flowing to the steering device in accordance with the operation of a handle. By changing the direction of the operating oil flowing to the steering actuator, the steering actuator extends or contracts, so that the running direction of the construction vehicle changes.
The construction vehicle includes a work machine cylinder, and for example, a work machine, such as a bucket, can be moved up and down by supplying the operating oil to the work machine cylinder. The work machine cylinder is connected to the hydraulic pump through a work machine control valve. The work machine control valve changes the direction of the operating oil flowing to the work machine in accordance with the operation of a lever, and by changing the direction of the operating oil flowing to the work machine cylinder, the work machine moves up or down.
Used as the work machine control valve is an open-center flow rate control valve. At a neutral position, the open-center flow rate control valve connects the hydraulic pump and a tank through a restrictor. Adopted as the hydraulic pump is a variable displacement hydraulic pump, and the hydraulic pump controls an ejection amount thereof in accordance with pressure generated at an upstream side of the restrictor.
The steering control valve and the work machine control valve are connected to a priority valve in parallel and further connected to the hydraulic pump through the priority valve. When driving the steering device, the priority valve preferentially supplies an operating fluid from the hydraulic pump to the steering control valve (see PTL 1, for example).